140 J. ION EXCHANGE

Article

The Inhibition of Fluoride Elution from Industrial Wastes with Portland Cement, Calcium and Magnesium Salts in Alkaline Region

Xiaoxu KUANG1*, Atsushi SASAKI2 and Masatoshi ENDO1

1Department of Chemistry and Chemical Engineering, Graduate School of Science and Engineering,

Yamagata University, 4-3-16 Jyonan, Yonezawa, Yamagata 992-8510, Japan 2 Department of Chemistry and Chemical Engineering, Faculty of Engineering, Yamagata University,

4-3-16 Jyonan, Yonezawa, Yamagata 992-8510, Japan

(Manuscript received March 31, 2018; accepted June 1, 2018)

Abstract

Recycling of fluoride industrial wastes is difficult to dispose, since the fluoride ions eluted again easily.

In this study, newly effective inhibition method in both neutral and alkaline region for fluoride elution

was investigated. Fluoride elution concentration from CaF2 was 9.8 mg/L at pH 8.1 and 288.5 mg/L at pH

12.2 with water/solid ratio of 10. Additions of Ca(OH)2, CaCl2 and NH4H2PO4 could restrain fluoride

elution concentration of CaF2 to 0.22 mg/L in the neutral region. This inhibition of fluoride elution was

due to a generation of hydroxyapatite (HAp) or chlorapatite (ClAp) which has high ion exchange capacity.

Additions of Portland cement, Ca(OH)2 and MgCl2 could restrain fluoride elution concentration of CaF2

to 0.47 mg/L in alkaline region. It was only 0.16% of 288.5mg/L which was the fluoride elution

concentration from CaF2 at pH 12.2. The elution of fluoride ions was restrained by the coprecipitating of

CaF2 with the high content of Ca2+ provided from Ca(OH)2 and cement hydrates. The carbonation of Ca2+

was prevented by the addition of Mg2+. Moreover, the forming of ettringite with a high ion-exchange

capacity and the solidification effect of Ca-bearing hydrates contributed to the inhibition of the elution of

fluoride. These additives for the practical industrial wastes, such as paper sludge and coal ash, with high

concentration of fluoride were conducted. The result indicates that the fluoride elution could be restrained

to meet the environmental standard (0.8 mg/L) in Japan in alkaline region. This inhibition method for

fluoride elution would be able to contribute to promotion of recycling of fluoride industrial wastes.

Keywords: Inhibition of fluoride elution, Recycling of fluoride wastes, Hydroxyapatite, Cement hydrates,

Alkaline region

1. Introduction

Fluorine, as an element of the halogen group, is widely

applied in semiconductors, medicines, glass and other

industries. To meet the growing demands for fluoride, the

productions of fluoride compounds have been also increasing.

It is known that excessive intake of fluoride could lead to the

disease of bone and teeth1). Recently, more and more reports

* Corresponding author E-mail: kuangmeinv@hotmail.com

indicate that fluoride could affect the brain and spinal cord,

causing various diseases and even life-threatening ones2-6).

Treatment of fluoride as a significant industrial pollutant has

become an important issue. Fluoride industrial wastes, such as

fluoride-containing sludge, coal ash and so on, occur

mainlyduring the treatment of fluoride wastewater by co-

precipitating with calcium and aluminum salt and fluoride

（ ）100

Vol.29 No.3 (2018) 141

Article

The Inhibition of Fluoride Elution from Industrial Wastes with Portland Cement, Calcium and Magnesium Salts in Alkaline Region

Xiaoxu KUANG1*, Atsushi SASAKI2 and Masatoshi ENDO1

1Department of Chemistry and Chemical Engineering, Graduate School of Science and Engineering,

Yamagata University, 4-3-16 Jyonan, Yonezawa, Yamagata 992-8510, Japan 2 Department of Chemistry and Chemical Engineering, Faculty of Engineering, Yamagata University,

4-3-16 Jyonan, Yonezawa, Yamagata 992-8510, Japan

(Manuscript received March 31, 2018; accepted June 1, 2018)

Abstract

Recycling of fluoride industrial wastes is difficult to dispose, since the fluoride ions eluted again easily.

In this study, newly effective inhibition method in both neutral and alkaline region for fluoride elution

was investigated. Fluoride elution concentration from CaF2 was 9.8 mg/L at pH 8.1 and 288.5 mg/L at pH

12.2 with water/solid ratio of 10. Additions of Ca(OH)2, CaCl2 and NH4H2PO4 could restrain fluoride

elution concentration of CaF2 to 0.22 mg/L in the neutral region. This inhibition of fluoride elution was

due to a generation of hydroxyapatite (HAp) or chlorapatite (ClAp) which has high ion exchange capacity.

Additions of Portland cement, Ca(OH)2 and MgCl2 could restrain fluoride elution concentration of CaF2

to 0.47 mg/L in alkaline region. It was only 0.16% of 288.5mg/L which was the fluoride elution

concentration from CaF2 at pH 12.2. The elution of fluoride ions was restrained by the coprecipitating of

CaF2 with the high content of Ca2+ provided from Ca(OH)2 and cement hydrates. The carbonation of Ca2+

was prevented by the addition of Mg2+. Moreover, the forming of ettringite with a high ion-exchange

capacity and the solidification effect of Ca-bearing hydrates contributed to the inhibition of the elution of

fluoride. These additives for the practical industrial wastes, such as paper sludge and coal ash, with high

concentration of fluoride were conducted. The result indicates that the fluoride elution could be restrained

to meet the environmental standard (0.8 mg/L) in Japan in alkaline region. This inhibition method for

fluoride elution would be able to contribute to promotion of recycling of fluoride industrial wastes.

Keywords: Inhibition of fluoride elution, Recycling of fluoride wastes, Hydroxyapatite, Cement hydrates,

Alkaline region

1. Introduction

Fluorine, as an element of the halogen group, is widely

applied in semiconductors, medicines, glass and other

industries. To meet the growing demands for fluoride, the

productions of fluoride compounds have been also increasing.

It is known that excessive intake of fluoride could lead to the

disease of bone and teeth1). Recently, more and more reports

* Corresponding author E-mail: kuangmeinv@hotmail.com

indicate that fluoride could affect the brain and spinal cord,

causing various diseases and even life-threatening ones2-6).

Treatment of fluoride as a significant industrial pollutant has

become an important issue. Fluoride industrial wastes, such as

fluoride-containing sludge, coal ash and so on, occur

mainlyduring the treatment of fluoride wastewater by co-

precipitating with calcium and aluminum salt and fluoride

（ ）100

waste products. Since fluoride easily eluted from the fluoride

industrial wastes, the disposal and/or recycle of those industrial

wastes also become difficult. Hydroxyapatite [HAp,

Ca10(PO4)6(OH)2] or chlorapatite [ClAp, Ca10(PO4)6Cl2] has

been reported to be able to remove fluoride ions in aqueous

solution because of its low solubility and excellent ion

exchange capability7-9). Moreover, hydrated cement containing

substantial amounts of Ca-bearing hydrates, such as portlandite

and ettringite, was also reported to be able to remove fluoride

ions by absorbing fluoride and co-precipitation10-13). However,

these studies focused on only removing of fluoride ions from

aqueous solutions and didn’t investigate an inhibition of

fluoride elution from the fluoride industrial wastes.

Additionally, reports about inhibition techniques of fluoride

eluted from the industrial wastes are also very few. Therefore,

the inhibition of fluoride eluted from the industrial wastes

needs further consideration. This study is aimed to develop an

effective inhibition method for fluoride elution. Effects of

forming of hydroxyapatite and cement hydrates and additions

of calcium and magnesium salts on an inhibition of fluoride

elution were studied. Additionally, tests of our fluoride

inhibition methods for paper sludge and coal ash as practical

industrial wastes, were also conducted from the viewpoints of a

practical application.

2. Experimental

2.1 Materials

These experiments described 2.2.1-2.2.3 with pure reagent

CaF2 (Wako Pure Chemical Industries Ltd., Japan) were

conducted for a simulation of fluoride elution from wastes.

Paper sludge (supplied by Nippon Paper Industries Co., Ltd.,

Japan) and coal ash (supplied by Fujico Co., Ltd.) were used as

source of fluoride wastes to test fluoride inhibitory effects.

The additives used to restrain the elution of fluoride were

Ca(OH)2 (99.9%), CaCl2·2H2O (99.0~103%), NH4H2PO4

(99.9%), Mg(OH)2 (96.0%), MgCl2·6H2O (99.0%) supplied by

Kanto Chemical Co., Inc, Japan and Portland cement supplied

by Sumitomo Osaka Cement Co., Ltd., Japan.

The equipment used in this study were Hitachi U-3000

Spectrophotometer, Rigaku Multipurpose X-ray diffraction

spectrometer (XRD Ultima IV) and Hitachi Field Emission

scanning electron microscope (SEM, SU8000).

2.2 The elution of fluoride ions from CaF2 pure reagent

CaF2 (10 g) was prepared by mixing with 3 mL of different

concentration of HCl or NaOH solution to adjusted initial pH

value of eluted solution to 4.6~12.2. They were placed at room

temperature for 6 h and then, dried with a muffle furnace

(Carbolite ESF3) at 60°C for 18 h. The dried samples were

conducted to fluoride elution test.

In this study, fluoride elution concentrations of the filtrates

were determined by measurements of fluoride concentration of

the filtrates with Lanthanum-alizarin complex

spectrophotometric method (ALC) using Hitachi U-3000

Spectrophotometer. These experiments were according to the

testing method for industrial wastewater of Japan (JIS K

0102)14). pH measurements of the filtrates were performed with

a Horiba pH Meter F-22. The crystal structure and phase

composition of dried mixtures were characterized by XRD. The

microstructures of dried mixtures were monitored by SEM.

2.3 Fluoride inhibition effect in neutral region

CaF2 (10 g) was mixed with 1.0 g of Ca(OH)2, 2.0 g of

CaCl2·2H2O, 0.78 g of NH4H2PO4. In this procedure, Ca(OH)2,

CaCl2·2H2O and NH4H2PO4 were added so that the molar ratio

of calcium/ phosphate was equal to 2. The pH values of eluted

solution of samples were adjusted to 3.6~12.7 by additions of

HCl or NaOH solution. The following experimental process

was the same as one in Section 2.2.

2.4 Fluoride inhibition effect in alkaline region

CaF2 (7 g) was mixed with 0.5 g of Ca(OH)2, 0.1~0.6 g of

MgCl2·6H2O and 3.0 g of ordinary Portland cement. Ultrapure

water was added to mixed samples, so that the water-to-solid

sample ratio was 0.3:1. The following experimental process

was the same as one in Section 2.2.

The experimental process of inhibition of fluoride elution

from CaF2 mixed with Ca(OH)2, Ca(OH)2 and Mg(OH)2, and

Portland cement respectively was the same as one descripted

above.

2.5 Application for practical wastes

Paper sludge and coal ash were mixed with CaF2 at weight

ratio of 9:1, respectively. Then Portland cement, Ca(OH)2 and

MgCl2·6H2O were added to these wastes to test fluoride

inhibitory effects. The following experimental process was the

same as one in Section 2.2.

3. Results and Discussion

3.1 Fluoride inhibition effect in neutral region

Figure 1 shows effect of pH for fluoride elution

concentration from CaF2 with and without additions of

Ca(OH)2/CaCl2 and NH4H2PO4. The fluoride elution

concentration from CaF2 without additives is 9.8 mg/L (pH=8.1)

in the neutral region. And then, it rapidly increases when the

pH value less than 8 or over 10. It could reach to 29.9 mg/L at

pH 4.6 and 288.5 mg/L at pH 12.2. The change in fluoride

elution concentration could be explained by the following

chemical equations:

CaF2 Ca2+ + 2F-

Ksp, CaF2 = [Ca2+][F-]2 = 3.95×10-11 (1)

H+ + F- HF

Ka, HF = [H+][F-]/[HF] = 3.5×10-4 (2)

Ca2+ + 2OH- Ca(OH)2

Ksp, Ca(OH)2 = [Ca2+][OH-]2 = 5.02×10-6 (3)

where Ka is acid dissociation constant, Ksp is solubility product.

（ ）101

142 J. ION EXCHANGE

[H+], [F-], [Ca2+] and [OH-] are activities of H+, F-, Ca2+ and

OH-, respectively. According to the Eqs. (1) and (2), CaF2 is

easy to be decomposed into calcium ions and fluoride ions in

the acid condition. With the increase of [H+], the ionization

equilibrium of HF moves to the right, F- has been consumed,

promoting the decomposition of CaF2. It ultimately leads to the

increase of concentration of fluoride ions in the solution.

According to the Eq. (3), with the increase in pH, the reaction

equilibrium of Ca(OH)2 moves to the right, Ca2+ has been

consumed. Consequently, this consuming of Ca2+ leads to

increase concentration of fluoride by the Eq. (3).

Fig. 1 Effect of pH for fluoride elution concentration from CaF2 with and without an addition of Ca(OH)2/CaCl2 and NH4H2PO4.

Generation of hydroxyapatite and chlorapatite reported to be

accelerated at Ca/P molar ratio of 1.67 or more15,16). Therefore,

the Ca/P molar ratio of Ca(OH)2/CaCl2 and NH4H2PO4 in this

study was set to 2. When Ca(OH)2/CaCl2 and NH4H2PO4 were

added, the lowest fluoride elution concentration was found at

pH 8.5 and was 0.22 mg/L which is below the concentration

specified in the environment standard of Japan. This fluoride

elution concentration: 0.22 mg/L, was only 2.3% of one

without the additions: 9.8 mg/L at pH 8.1. The elution of

fluoride was restrained by a large amount of Ca2+ dissolved

from Ca(OH)2/CaCl2 and the products ClAp and HAp. The

chemical reaction equation postulated among Ca(OH)2/CaCl2,

NH4H2PO4 and CaF2 are shown in Eqs. (4) -(7).

10Ca2+ + 6PO43- + 2Cl- Ca10(PO4)6Cl2 (4)

10Ca2+ + 6PO43- + 2OH- Ca10(PO4)6(OH)2 (5)

Ca10(PO4)6(OH)2 + 2F- Ca10(PO4)6F2 + 2OH- (6)

Ca10(PO4)6Cl2 + 2F- Ca10(PO4)6F2 + 2Cl- (7)

Hydroxyapatite (HAp, Ca10(PO4)6(OH)2) and/or chlorapatite

(ClAp, Ca10(PO4)6Cl2) would be formed by the reaction of Eqs.

(4) and (5). Additionally, its precursors, tricalcium phosphate

[Ca3(PO4)2] and/or dicalcium phosphate [CaHPO4], would be

also formed. It was reported that hydroxide ions (OH-) or

chloride ion (Cl-) of those apatites and its precursors restrained

the fluoride elution by exchanging with the free fluoride ions.

Moreover, those apatites or its precursors probably restrained

more the elution of fluoride ions with wrapping on the surface

of fluoride compound during its forming. These inhibition

behaviors of apatites and its precursors were shown in Fig. 2.

Fig. 2 Schematic illustration for the inhibition of fluoride with hydroxyapatite (HAp) or chlorapatite (ClAp), or its precursors (Ca3(PO4)2 and/or CaHPO4).

Fig. 3 XRD patterns of CaF2 which mixed with (a) CaCl2 and

NH4H2PO4 at pH 3.6, and (b) Ca(OH)2 and NH4H2PO4 at pH 8.5.

The reflections assigned to calcium hydrogen chloride

phosphate hydrate (CaClH2PO4·H2O, ICSD file No.00-044-

0746), chlorapatite (Ca10(PO4)6Cl2, ICSD file No.00-027-0074)

and hydroxyapatite (Ca10(PO4)6(OH)2, ICSD file No. 00-009-

0432) were observed in Fig. 3. The result suggests that HAp

and ClAp had been generating from the reactions of Ca(OH)2,

CaCl2 and NH4H2PO4. The dissolution of Ca(OH)2/CaCl2 and

the generation of ClAp and HAp decreased the fluoride elution

concentration from CaF2 below the environmental standards in

the neutral region. However, the fluoride elution concentration

was still reaches to 196.9 mg/L at pH 12.7 and far more than

environmental standards in the alkaline region. It was because

that, in alkaline region, Ca2+ was consumed not only by OH- to

generate Ca(OH)2 (Eq. (3)), but also by PO43- to generate HAp

(Eq. (5)). The consuming of large amounts of Ca2+ contributes

to the elution of fluoride. On the other hand, because of the

similarity in OH- and F- in charge, ionic radius and adsorption

sites in apatite, OH- will compete with F-, resulting into the

050100150200250300350

2 4 6 8 10 12 14

F co

nc. [

mg/

L]

pH

Without the additives With the additives

Calcium fluoride Portlandite ●Hydroxyapatite▼Chlorapatite

Calcium hydrogen chloride phosphate hydrate

（ ）102

Vol.29 No.3 (2018) 143

[H+], [F-], [Ca2+] and [OH-] are activities of H+, F-, Ca2+ and

OH-, respectively. According to the Eqs. (1) and (2), CaF2 is

easy to be decomposed into calcium ions and fluoride ions in

the acid condition. With the increase of [H+], the ionization

equilibrium of HF moves to the right, F- has been consumed,

promoting the decomposition of CaF2. It ultimately leads to the

increase of concentration of fluoride ions in the solution.

According to the Eq. (3), with the increase in pH, the reaction

equilibrium of Ca(OH)2 moves to the right, Ca2+ has been

consumed. Consequently, this consuming of Ca2+ leads to

increase concentration of fluoride by the Eq. (3).

Fig. 1 Effect of pH for fluoride elution concentration from CaF2 with and without an addition of Ca(OH)2/CaCl2 and NH4H2PO4.

Generation of hydroxyapatite and chlorapatite reported to be

accelerated at Ca/P molar ratio of 1.67 or more15,16). Therefore,

the Ca/P molar ratio of Ca(OH)2/CaCl2 and NH4H2PO4 in this

study was set to 2. When Ca(OH)2/CaCl2 and NH4H2PO4 were

added, the lowest fluoride elution concentration was found at

pH 8.5 and was 0.22 mg/L which is below the concentration

specified in the environment standard of Japan. This fluoride

elution concentration: 0.22 mg/L, was only 2.3% of one

without the additions: 9.8 mg/L at pH 8.1. The elution of

fluoride was restrained by a large amount of Ca2+ dissolved

from Ca(OH)2/CaCl2 and the products ClAp and HAp. The

chemical reaction equation postulated among Ca(OH)2/CaCl2,

NH4H2PO4 and CaF2 are shown in Eqs. (4) -(7).

10Ca2+ + 6PO43- + 2Cl- Ca10(PO4)6Cl2 (4)

10Ca2+ + 6PO43- + 2OH- Ca10(PO4)6(OH)2 (5)

Ca10(PO4)6(OH)2 + 2F- Ca10(PO4)6F2 + 2OH- (6)

Ca10(PO4)6Cl2 + 2F- Ca10(PO4)6F2 + 2Cl- (7)

Hydroxyapatite (HAp, Ca10(PO4)6(OH)2) and/or chlorapatite

(ClAp, Ca10(PO4)6Cl2) would be formed by the reaction of Eqs.

(4) and (5). Additionally, its precursors, tricalcium phosphate

[Ca3(PO4)2] and/or dicalcium phosphate [CaHPO4], would be

also formed. It was reported that hydroxide ions (OH-) or

chloride ion (Cl-) of those apatites and its precursors restrained

the fluoride elution by exchanging with the free fluoride ions.

Moreover, those apatites or its precursors probably restrained

more the elution of fluoride ions with wrapping on the surface

of fluoride compound during its forming. These inhibition

behaviors of apatites and its precursors were shown in Fig. 2.

Fig. 2 Schematic illustration for the inhibition of fluoride with hydroxyapatite (HAp) or chlorapatite (ClAp), or its precursors (Ca3(PO4)2 and/or CaHPO4).

Fig. 3 XRD patterns of CaF2 which mixed with (a) CaCl2 and

NH4H2PO4 at pH 3.6, and (b) Ca(OH)2 and NH4H2PO4 at pH 8.5.

The reflections assigned to calcium hydrogen chloride

phosphate hydrate (CaClH2PO4·H2O, ICSD file No.00-044-

0746), chlorapatite (Ca10(PO4)6Cl2, ICSD file No.00-027-0074)

and hydroxyapatite (Ca10(PO4)6(OH)2, ICSD file No. 00-009-

0432) were observed in Fig. 3. The result suggests that HAp

and ClAp had been generating from the reactions of Ca(OH)2,

CaCl2 and NH4H2PO4. The dissolution of Ca(OH)2/CaCl2 and

the generation of ClAp and HAp decreased the fluoride elution

concentration from CaF2 below the environmental standards in

the neutral region. However, the fluoride elution concentration

was still reaches to 196.9 mg/L at pH 12.7 and far more than

environmental standards in the alkaline region. It was because

that, in alkaline region, Ca2+ was consumed not only by OH- to

generate Ca(OH)2 (Eq. (3)), but also by PO43- to generate HAp

(Eq. (5)). The consuming of large amounts of Ca2+ contributes

to the elution of fluoride. On the other hand, because of the

similarity in OH- and F- in charge, ionic radius and adsorption

sites in apatite, OH- will compete with F-, resulting into the

050100150200250300350

2 4 6 8 10 12 14

F co

nc. [

mg/

L]

pH

Without the additives With the additives

Calcium fluoride Portlandite ●Hydroxyapatite▼Chlorapatite

Calcium hydrogen chloride phosphate hydrate

（ ）102

decrease in fluoride inhibitory effect in alkaline region7,8,17,18).

Consequently, more effective fluoride inhibition method in the

alkaline region is needed.

3.2 Fluoride inhibition effect in alkaline region

The fluoride elution concentration was decreased

significantly with the increase in the addition amount of

Ca(OH)2 (Table 1). When the addition amount of Ca(OH)2 was

over 0.7 g, the fluoride concentration could be reduced to less

than 4 mg/L, this concentration was only 1.38% of 288.5 mg/L

which was the fluoride elution concentration from CaF2 at pH

12.2. A large amount Ca2+ provided from Ca(OH)2 would

inhibit the elution of fluoride ions from CaF2 by the reactions

according to Eqs. (1) and (3). However, the fluoride

concentration had been hardly satisfied the environmental

standards only by the addition with Ca(OH)2.

Table 1 Effect of Ca(OH)2 for fluoride elution concentration

from 10.0 g of CaF2.

Table 2 Effect of Mg(OH)2 for fluoride elution concentration

from 10.0 g of CaF2 and 1.0 g of Ca(OH)2 with heating

at 60°C or vacuum drying at room temperature.

A decrease in concentration of fluoride ions was observed

with the increase in the addition amount of Mg(OH)2 with both

heat drying and vacuum drying methods (Table 2). It was

considered that the elution of fluoride ions would be restrained

by the addition of Ca(OH)2 and Mg(OH)2, since Ca2+ and Mg2+

provided from Ca(OH)2 and Mg(OH)2 could react with free F-

to generate CaF2 and MgF2. The fluoride elution concentration

of CaF2 with vacuum drying at room temperature was lower

than that with heating at 60°C. The reflections assigned to

portlandite (Ca(OH)2, ICSD file No.00-004-0733) and calcite

(CaCO3, ICSD file No.01-072-1652) were observed at the

condition with an addition of Ca(OH)2 and heating at 60°C (Fig.

4). Brucite (Mg(OH)2, ICSD file No.00-007-0239) was

appeared when Mg(OH)2 was added. On another hand, at the

conditions with vacuum drying at room temperature, the

reflection assigned to calcite was disappeared. Furthermore, the

reflection assigned to portlandite became stronger. The Ca2+

consumption from carbonation of Ca2+ by reacting with CO2 in

the air could be controlled in vacuum drying condition. The

decrease in the elution concentration of fluoride under vacuum

drying condition indicates that the inhibition of the carbonation

of Ca2+ could be a way to promote the inhibitory effect of

elution of fluoride ions.

Fig. 4 XRD patterns of CaF2 which mixed with (a) Ca(OH)2

(b) Ca(OH)2, and 0.5 g of Mg(OH)2 with heating at

60°C (c) Ca(OH)2 and (d) Ca(OH)2 and 0.5 g of Mg(OH)2

with vacuum drying at room temperature.

Table 3 Effect of ordinary Portland cement for fluoride

elution concentration from 7.0 g of CaF2.

Fig. 5 XRD pattern of CaF2 which mixed with ordinary

Portland cement at weight ratio of 50%.

A significant decrease in fluoride elution concentration was

observed with an increase in an addition amount of Portland

cement (Table 3). The lowest fluoride elution concentration:

0.68 mg/L was found when the weight ratio of Portland cement

was 50%, it was only 0.24% of 288.5 mg/L which was the

fluoride elution concentration from CaF2 at pH 12.2. The

reflections assigned to portlandite (Ca(OH)2, ICSD file No.01-

Addition amount of Ca(OH)2 (g)

0.2 0.4 0.5 0.7 1.0

F conc. [mg/L] 75.8 73.9 38.2 3.3 3.9

Drying method

Addition amount of Mg(OH)2 (g)

0.0 0.1 0.3 0.5

F conc. [mg/L] vacuum 12.4 11.3 9.2 8.7 F conc. [mg/L] oven 13.8 14.3 13.4 11.0

Addition of Portland cement (wt.%)

10 20 30 40 50

F conc. [mg/L] 3.9 1.7 1.2 0.86 0.68

ΔPortlandite ○Calcite Magnesium hydroxide

(d)

(c)

(b)

(a)

ΔPortlandiite ○Calcite ■Ettringite ▼Calcium silicate hydrate

（ ）103

144 J. ION EXCHANGE

076-0571), calcite (CaCO3, ICSD file No.00-005-0586),

calcium silicate hydrate (Ca3SiO5, ICSD file No.01-086-0402)

and ettringite (Ca6Al2(SO4)3(OH)12·26H2O, ICSD file No.00-

041-1451) were observed in Fig. 5. The elution of fluoride ions

was restrained by the coprecipitating of CaF2 with the high

content of Ca2+ provided from cement hydrates. In addition, the

forming of ettringite with high ion-exchange capacity and the

solidification effect of Ca-bearing hydrates contributes to the

inhibition of the elution of fluoride.

Table 4 Effect of MgCl2 for fluoride elution concentration

from 7.0 g of CaF2, Ca(OH)2 and Portland cement

under the drying conditions at 60°C or room

temperature.

A: 3.0 g of Portland cement and 0.5 g of Ca(OH)2; B: MgCl2

All pH values of samples in Table 4 were above 12.2. The

fluoride elution concentration was restrained to less than 1.5

mg/L with an addition of Portland cement with both drying

conditions. Then, it slightly decreased with an increase in

addition amounts of Ca(OH)2 and MgCl2·6H2O. The highest

effect on an inhibition of a fluoride elution was found when the

samples were mixed with Portland cement, Ca(OH)2 and 0.4 g

of MgCl2·6H2O. Those fluoride elution concentrations were

restrained to 0.78 mg/L with drying at 60°C and 0.47 mg/L at

room temperature. In addition, the samples treated with drying

at the room temperature showed a higher effect in an inhibition

of fluoride elution compared with ones with drying at 60°C.

The reflections assigned to portlandite (Ca(OH)2, ICSD file

No.00-004-0733), calcite (CaCO3, ICSD file No.00-005-0586)

and calcium silicate hydrate (Ca3SiO5, ICSD file No. 01-086-

0402) were observed at all the experimental conditions (Fig. 6).

The reflections assigned to portlandite as cement hydrates

became stronger when Ca(OH)2 was added. The reflections

assigned to portlandite, calcite and calcium silicate hydrate

weakened clearly when addition amounts of MgCl2 increased,

while gypsum appeared. Mg2+ was reported to have an

inhibitory effect on the growth of calcite in many researches. In

a saturated calcite solution, the presence of Mg2+ can enhance

the solubility of calcite by incorporating into the calcite lattice

even at low Mg concentration or aqueous Mg/Ca ratio19-22).

Actually, the peak of reflection assigned to calcite decreased

with an increase in an addition of MgCl2. Additionally, in the

coexistence of MgCl2 and Ca(OH)2 of Portland cement paste,

Mg2+ was reported to react with hydroxyl ions to generate the

precipitation of Mg(OH)2 because of its low solubility,

Fig. 6 XRD patterns of CaF2 which mixed with (a) Portland

cement; (b) Portland cement and Ca(OH)2; (c) Portland

cement, Ca(OH)2 and 0.4g of MgCl2; (d) Portland

cement, Ca(OH)2 and 0.4 g of MgCl2 with drying at 60°C

and (e) Portland cement, Ca(OH)2 and 0.4 g of MgCl2

with drying at room temperature.

(a)

(b)

Fig. 7 SEM imagines for (a) cement paste powder and (b)

ettringite from dried samples which mixed with

Portland cement, Ca(OH)2 and 0.4 g of MgCl2.

Additives

Drying condition

60°C Room Temp.

F conc.

[mg/L]

pH [-] F conc.

[mg/L]

pH [-]

cement 1.44 12.5 1.48 12.4

A 1.18 12.5 1.08 12.5

A+0.1 g of B 1.21 12.5 1.12 12.5

A+0.2 g of B 1.24 12.3 0.96 12.4

A+0.4 g of B 0.78 12.3 0.47 12.4

A+0.6 g of B 0.98 12.2 0.74 12.3

Ettringite

1.0μm

15.0μ

ΔPortlandiite ○Calcite □Gypsum

▼Calcium silicate hydrate

(e)

(d)

(c)

(b)

(a)

（ ）104

Vol.29 No.3 (2018) 145

076-0571), calcite (CaCO3, ICSD file No.00-005-0586),

calcium silicate hydrate (Ca3SiO5, ICSD file No.01-086-0402)

and ettringite (Ca6Al2(SO4)3(OH)12·26H2O, ICSD file No.00-

041-1451) were observed in Fig. 5. The elution of fluoride ions

was restrained by the coprecipitating of CaF2 with the high

content of Ca2+ provided from cement hydrates. In addition, the

forming of ettringite with high ion-exchange capacity and the

solidification effect of Ca-bearing hydrates contributes to the

inhibition of the elution of fluoride.

Table 4 Effect of MgCl2 for fluoride elution concentration

from 7.0 g of CaF2, Ca(OH)2 and Portland cement

under the drying conditions at 60°C or room

temperature.

A: 3.0 g of Portland cement and 0.5 g of Ca(OH)2; B: MgCl2

All pH values of samples in Table 4 were above 12.2. The

fluoride elution concentration was restrained to less than 1.5

mg/L with an addition of Portland cement with both drying

conditions. Then, it slightly decreased with an increase in

addition amounts of Ca(OH)2 and MgCl2·6H2O. The highest

effect on an inhibition of a fluoride elution was found when the

samples were mixed with Portland cement, Ca(OH)2 and 0.4 g

of MgCl2·6H2O. Those fluoride elution concentrations were

restrained to 0.78 mg/L with drying at 60°C and 0.47 mg/L at

room temperature. In addition, the samples treated with drying

at the room temperature showed a higher effect in an inhibition

of fluoride elution compared with ones with drying at 60°C.

The reflections assigned to portlandite (Ca(OH)2, ICSD file

No.00-004-0733), calcite (CaCO3, ICSD file No.00-005-0586)

and calcium silicate hydrate (Ca3SiO5, ICSD file No. 01-086-

0402) were observed at all the experimental conditions (Fig. 6).

The reflections assigned to portlandite as cement hydrates

became stronger when Ca(OH)2 was added. The reflections

assigned to portlandite, calcite and calcium silicate hydrate

weakened clearly when addition amounts of MgCl2 increased,

while gypsum appeared. Mg2+ was reported to have an

inhibitory effect on the growth of calcite in many researches. In

a saturated calcite solution, the presence of Mg2+ can enhance

the solubility of calcite by incorporating into the calcite lattice

even at low Mg concentration or aqueous Mg/Ca ratio19-22).

Actually, the peak of reflection assigned to calcite decreased

with an increase in an addition of MgCl2. Additionally, in the

coexistence of MgCl2 and Ca(OH)2 of Portland cement paste,

Mg2+ was reported to react with hydroxyl ions to generate the

precipitation of Mg(OH)2 because of its low solubility,

Fig. 6 XRD patterns of CaF2 which mixed with (a) Portland

cement; (b) Portland cement and Ca(OH)2; (c) Portland

cement, Ca(OH)2 and 0.4g of MgCl2; (d) Portland

cement, Ca(OH)2 and 0.4 g of MgCl2 with drying at 60°C

and (e) Portland cement, Ca(OH)2 and 0.4 g of MgCl2

with drying at room temperature.

(a)

(b)

Fig. 7 SEM imagines for (a) cement paste powder and (b)

ettringite from dried samples which mixed with

Portland cement, Ca(OH)2 and 0.4 g of MgCl2.

Additives

Drying condition

60°C Room Temp.

F conc.

[mg/L]

pH [-] F conc.

[mg/L]

pH [-]

cement 1.44 12.5 1.48 12.4

A 1.18 12.5 1.08 12.5

A+0.1 g of B 1.21 12.5 1.12 12.5

A+0.2 g of B 1.24 12.3 0.96 12.4

A+0.4 g of B 0.78 12.3 0.47 12.4

A+0.6 g of B 0.98 12.2 0.74 12.3

Ettringite

1.0μm

15.0μ

ΔPortlandiite ○Calcite □Gypsum

▼Calcium silicate hydrate

(e)

(d)

(c)

(b)

(a)

（ ）104

resulting in the decrease in pH value and an increase in the

solubility of Ca23,24). The reflection assigned to ettringite was

not observed. The addition weight ratio of Portland cement in

Fig. 6(a) was less than that of Fig. 5. Little amount of

production of ettringite caused this weak diffraction.

Coagulation of cement paste and the generation of ettringite

were confirmed in Fig. 7. Ettringite exhibit a rob or needle-like

particles with 1-2µm length. The elution of fluoride ions was

restrained by the coprecipitating of CaF2 with the high content

of Ca2+ provided from Ca(OH)2 and cement hydrates. The

carbonation of Ca2+ was prevented by the addition of Mg2+.

Moreover, the forming of ettringite with high ion-exchange

capacity and the solidification effect of Ca-bearing hydrates

contributed to the inhibition of the elution of fluoride. Water in

cement paste is more easily lost with drying conditions at 60°C.

The loss of water delayed hydration of cement paste, resulting

into a low solidification effect of cement paste. Additionally,

Ca(OH)2, added initially and generated in hydration process, is

easily to dissolve and react with CO2 in the air in the fluoride

elution experiment. Therefore, the fluoride inhibitory effect is

better with the drying at room temperature.

3.3 Application for practical wastes

The practical fluoride inhibition test for the addition of

Ca(OH)2, MgCl2 and Portland cement was conducted with

using of the paper sludge and the coal ash. The composition of

the two industrial wastes showed distinct differences (Table 5).

Paper sludge showed high content of SiO2 (32.44 wt.%), Al2O3

(7.28 wt.%), Fe2O3 (5.95wt.%), CaO (5.47 wt.%) and S (16.50

wt.%). As for coal ash, it consisted mainly of CaO (16.59

wt.%), MgO (7.50 wt.%), SiO2 (4.96 wt.%), Na2O (1.66wt.%),

S (20.50 wt.%) and Cl (4.66 wt.%). The initial fluoride

concentration eluted from paper sludge and coal ash without

CaF2 was 0.43 mg/L and 1.45 mg/L, respectively.

Table 5 Chemical compositions of paper sludge and coal

ash (wt.%).

ND: no detection

The paper sludge samples treated by CaF2 indicate paper

sludges mixed with CaF2 by the method described in Section

2.5. The fluoride elution concentration of this treated paper

sludge and coal ash samples without any additives were 3.51

mg/L (pH=8.3) and 1.2 mg/L (pH=12.0), respectively (Table

6). The fluoride elution concentrations from treated paper

sludge and coal ash samples were restrained to below

environmental standard 0.8 mg/L with an addition of only

Portland cement. The highest fluoride inhibitory effect was

found when treated paper sludge and coal ash mixed with 3.0 g

of cement, 0.5 g of Ca(OH)2 and 0.4 g of MgCl2. The fluoride

elution concentrations from paper sludge and coal ash samples

were 0.35 mg/L and below 0.1 mg/L.

Table 6 Fluoride concentration of treated paper sludge and

coal ash samples prepared with Portland cement,

Ca(OH)2 and MgCl2 with drying at 60°C. Paper sludge

and coal ash were mixed with CaF2 at weight ratio of

9:1. The addition amount of these fluoride wastes was

7.0 g.

Additives F conc. [mg/L]

paper sludge coal ash

none 3.51 1.2 cement 0.75 0.55 A 0.65 0.68 A+0.1 g of B 0.52 0.84 A+0.2 g of B 0.43 0.56 A+0.4 g of B 0.35 0.85

A+0.6 g of B 0.35 <0.10 A+0.8 g of B 0.37 – A+1.0 g of B 0.39 – A: 3.0 g of Portland cement and 0.5 g of Ca(OH)2; B: MgCl2; – : no determination.

These results indicate that the additives, Portland cement,

Ca(OH)2 and MgCl2, could be used for inhibition of fluoride

elution from industrial waste in alkaline region to meet the

environmental standard in Japan. The industrial wastes, paper

sludge and coal ash, could be reused for civil resource, such as

road base or ground consolidation after treatment.

4. Conclusion

The fluoride concentration eluted from CaF2 was inhibited

successfully in both neutral and alkaline region. In neutral

region, the additions of Ca(OH)2, CaCl2 and NH4H2PO4 was

able to restrained the fluoride elution concentration to 0.22

mg/L by a large amount of Ca2+ dissolved from Ca(OH)2/CaCl2

and the products HAp and ClAp. In alkaline region, the

additions of Portland cement, Ca(OH)2 and MgCl2 was able to

restrained the fluoride elution concentration to 0.78mg/L with

drying at 60°C and 0.47 mg/L at room temperature. The elution

Chemical compositions (wt.%)

Paper sludge Coal ash

SiO2 32.44 4.96Al2O3 7.28 0.73 FeO 0.12 0.05

Fe2O3 5.95 0.53 CaO 5.47 16.59K2O 1.25 0.62 MgO 0.98 7.50 Na2O 0.95 1.66

Cu 0.10 0.11Zn 0.33 0.31S 16.50 20.50Cl ND 4.66

Other 28.58 66.89

（ ）105

146 J. ION EXCHANGE

of fluoride ions was restrained by the coprecipitating of CaF2

with the high content of Ca2+ provided from Ca(OH)2 and

cement hydrates. The carbonation of Ca2+ was prevented by the

addition of Mg2+. Moreover, the forming of ettringite with high

ion-exchange capacity and the solidification effect of Ca-

bearing hydrates contributed to the inhibition of the elution of

fluoride.

Practical tests for the industrial wastes, paper sludge and coal

ash with high concentration of fluoride were conducted. The

result indicates that the fluoride elution concentration of these

industrial wastes could be restrained to meet the environmental

standard in Japan in alkaline region.

References

1) C. Death, G. Coulson, U. Kierdorf, H. Kierdorf, W.K.

Morris and J. Hufschmid, Sci Total. Environ., 533, 528

(2015).

2) O.L. Adebayo, P.D. Shallie, B.A. Salau, E.O. Ajani and

G.A. Adenuga, J. Trace Elem. Med. Biol., 27, 370 (2013).

3) M. Li, J. Cao, J. Song, B. Zhou, C. Feng and J. Wang,

Chemosphere, 145, 365 (2016).

4) F. Liu, J. Ma, H. Zhang, P. Liu, Y.P. Liu, B. Xing and

Y.H. Dang, Physiol. Behav., 124, 1 (2014).

5) Z. Sun, W. Zhang, X. Xue, Y. Zhang, R. Niu, X. Li, B. Li,

X. Wang and J. Wang, Chemosphere, 144, 1012 (2016).

6) M. Bartos, F. Gumilar, C. Bras, C.E. Gallegos, L.

Giannuzzi, L.M. Cancela and A. Minetti, Physiol. Behav.,

147, 205 (2015).

7) S. Gao, J. Cui and Z. Wei, J. Fluorine Chem., 130, 1035

(2009).

8) M. Jiménez-Reyes and M. Solache-Ríos, J. Hazard.

Mater., 180, 297 (2010).

9) D. Zhang, H. Luo, L. Zheng, K. Wang, H. Li, Y. Wang

and H. Feng, J. Hazard. Mater., 241-242, 418 (2012).

10) Y. Tsunashima, A. Iizuka, J. Akimoto, T. Hongo and A.

Yamasaki, Chem. Eng. J., 200-202, 338 (2012).

11) S. Kagne, S. Jagtap, P. Dhawade, S.P. Kamble, S. Devotta

and S.S. Rayalu, J. Hazard. Mater.,154, 88 (2008).

12) W.H. Kang, E.I. Kim and J.Y. Park, Desalination, 202, 38

(2007).

13) S. Bibi, A. Farooqi, K. Hussain and N. Haider, J. Clean.

Prod., 87, 882 (2015).

14) JIS K0102, “Testing Methods for Industrial Wastewater”,

Japanese Industrial Standard, Japanese Standards

Association (2016).

15) S. Kim, H.S. Ryu and H.S. Hong, Metals Mater. Int., 10,

171 (2004).

16) M.C. Wang, W.J. Shih, I.M. Hung, H.T. Chen, M.H. Hon

and H.H. Huang, Ceram. Int., 41, 1223 (2015).

17) J.M. Astilleros, L. Fernández-Díaz and A. Putnis, Chem.

Geol., 271, 52 (2010).

18) M. Hong, J. Xu, H.H. Teng, Geochim. Cosmochim. Acta,

172, 55 (2016).

19) Y. Nie, C. Hu and C. Kong, J. Hazard. Mater., 233-234,

194 (2012).

20) C.S. Sundaram, N. Viswanathan and S. Meenakshi, J.

Hazard. Mater., 155, 206 (2008).

21) L.C. Nielsen, J.J. De Yoreo and D.J. DePaolo, Geochim.

Cosmochim. Acta, 115, 100 (2013).

22) Y.P. Lin and P.C. Singer, J. Cryst. Growth, 312, 136

(2009).

23) K. De Weerdt, A. Colombo, L. Coppolab, H. Justnes and

M.R. Geiker, Cem. Concr. Res., 68, 196 (2015).

24) M. Maes and N. De Belie, Constr. Build. Mater., 155, 630

(2017).

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